The invention is based on a system for changing a signal representing the rotational speed of at least one, wheel of a motor vehicle. For the closed-loop or open-loop control of the braking force, drive power, and/or dynamics of motion of a motor vehicle, it is known that the rotational speeds of the wheels of the vehicle can be measured. The state of the art provides various methods (e.g., Hall or magnetoresistive sensors) for this purpose. In addition, it is known that the point at which the brake lining of a vehicle brake becomes worn out can be determined by, for example, embedding contact pins a certain depth below the surface of the brake lining. When the brake lining has been worn down to this depth, the pins trigger a contact.
Active sensors for use in open-loop or closed-loop antilock, drive slip, engine, and transmission control systems in motor vehicles are described in, for example, the article "Integrierte Hall-Effekt-Sensoren zur Positions-und Drehzahlerkennung", eletronik industrie, Vol. 7, pp. 29-31, 1995. In a two-wire system, sensors of this type supply two current levels, which are converted by a precision resistor into two voltage levels in a corresponding control device.
In addition to the Hall-effect sensors mentioned above, it is also possible to use magnetoresistive sensors to detect rotational speeds. This is known from, for example, the article "Neue, Alternative Losungen fur Drehzahlsensoren im Kraftfahrzeug auf magnetoresisitiver Basis", VDI-Berichte, No. 509, 1984.
A special shared device for detecting brake lining wear and the rotational speed of a wheel is described In DE-C2 26 06 012 (U.S. Pat. No. 4,076,440). For this purpose, the detected brake lining wear and the wheel speed detected by means of an inductive sensor are sent over a common signal line to an evaluation unit. This is achieved in that the wheel speed sensor is entirely or partially short-circuited in reaction to the detected amount of brake lining wear.
To detect the rotational speed of the wheel and the brake lining wear at the wheel brake, other systems, such as those described in DE-C 43 22 440, require at least two signal lines between the wheel unit and the evaluation unit.
In regard to the detection of rotational speed described above, it is known that the air gap between the rotating toothed wheel rim and the actual sensor element has a considerable effect on the quality of the rotational speed signal. In reference to this point, see, for example, DE-OS 32 01 811, for example.
In addition, in the case of systems to help the driver get the vehicle started (so-called "hill holders"), for example, information on the rotational direction of the wheels is also required. Here it is especially necessary to known if the vehicle is moving backwards. See, for example, DE-OS 35 10 651 on this point.
The information cited above and other types of data (such as data on the degree of brake lining wear, the size of the air gap, and the direction of rotation) are usually detected close to the wheel and evaluated in a control unit located some distance away from the wheel. The information must therefore be transmitted to the control unit.
In the case of an engine (internal combustion engine or electric motor), it is known that the rpm's of the engine can be detected by means of inductive, magnetoresistive, or Hall sensors.
The task of the present invention is to provide means for transmitting the rotational speed signal and additional information in the simplest possible, reliable manner.